It can help to clarify a goal. The very act of talking it out can help make it seem more real and help to identify the best way to go about it. If the person listening asks some insightful questions, all the better.
Regardless of whether we like to admit it or not, we all like praise and getting a 'well done' or pat on the back is motivational.
When there is no one around to notice, or to say anything about an incomplete task, it's easy to push it to the back of our to-do list.
We don't like to look foolish so once we've told someone we are going to do something, the idea of having to tell them we haven't done it can help to keep us motivated when times get tough.
Accountability works! A deadline to which we are held accountable works even better. As every student knows there's nothing like a looming 'submit' deadline to focus the mind on a project. Finding someone who will hold us accountable to do the things we've DECIDED WE WANT TO DO, can be very beneficial.